The broad, long-term objective of this research is to determine if nitric oxide (NO) mediates opioid dependence. This application focuses on the effects of NO synthase (NOS) inhibitors and NO donors on morphine physical dependence in male rats. Preliminary studies indicated that central depressant actions of NOS inhibitors decrease the severity of morphine withdrawal, that NO donors induce morphine- suppressed withdrawal signs, and that chronic NOS inhibition induces a morphine-like dependence. Two specific aims are proposed to test the hypothesis that NO mediates part of the expression of morphine withdrawal and that NOS inhibition mediates part of the development of morphine physical dependence: First, studies to determine if NO mediates the expression of morphine abstinence will be carried out by implanting male rats with morphine pellets chronically, then administering combinations of NO-related agents and measuring abstinence signs after naloxone- or NO donor- precipitated withdrawal, or after abrupt, spontaneous withdrawal. Second, the hypothesis that NOS inhibition mediates the development of morphine physical dependence will be tested in: (1) chronic morphine studies, (2) chronic NOS inhibition studies, and (3) studies measuring morphine levels. These experiments will include naloxone- and/or NO- precipitated or spontaneous withdrawal after several combinations of treatments to examine how the opioid and NO systems interact in dependence development. Treatments will include (1) chronic morphine pellets and osmotic pumps filled with a NOS inhibitor or NOS substrates, and (2) chronic repeated administration of morphine, a NOS inhibitor, NOS substrates, and a NO donor followed by measures of withdrawal severity and morphine biodisposition. The health relatedness of this project includes the characterization of new physiological and pathological mechanisms of action of NO in opioid effects that may suggest new rationales for the treatment and prevention of opioid abuse and withdrawal and improvements in the medical use of opioids.